The ability of communication, navigation and a wide variety of frequency and timing measurement equipment to perform properly is determined in part by their phase noise performance. A common aspect of their design, manufacture, calibration, and ultimate use is phase modulation (PM) noise introduced by such equipment. While other performance parameters are certainly present and important, they are often relatively more simple to predict, calibrate and control than phase modulation noise performance. There are a variety of systems for measuring phase modulation noise, and these are discussed in the publications listed in the attached Appendix, incorporated herein by reference.
Currently, the most accurate method of measuring phase modulation noise is a system that includes a precision wide band phase modulator inserted in series with a signal whose PM noise is to be measured. The modulator is used to create phase modulation side bands about the carrier of the signal for determining the corrections to the measurement as a function of Fourier frequency offset from the carrier. The result is high accuracy and wide bandwidth of PM noise measurements. This system is disclosed in U.S. Pat. No. 4,968,908 (listed as No. 10 in the attached Appendix.
The output waveform of PM. noise is defined in the first publication listed in the Appendix. The amplitude fluctuations are contained in the term .epsilon.(t) and the phase or frequency fluctuations are contained in the term .phi.(t). Variations in the .phi.(t) value result in unequal spacing of the zero crossings of the waveform, while variations in the .epsilon.(t) value result in variations of the height of the peaks. Phase modulation noise is often defined by S.sub..phi. (f), which is a spectral density of phase fluctuations. The derivation of this term is found in the first four articles listed in the Appendix and can be expressed as: EQU S.sub..phi. (f)=.delta..phi..sup.2 (f)/BW units radians.sup.2 /Hz (1)
where .delta..phi..sup.2 (f) is the mean squared phase fluctuation measured at a frequency separation f from the carrier is a bandwidth BW.
Similarly, the practical definition for the spectral density of amplitude modulation noise is: EQU S.sub.a (f)=.delta.V.sup.2 (f)/((V.sub.o.sup.2)BW) units 1/Hz (2)
where .delta.V.sub.o.sup.2 (f) is the mean squared voltage fluctuation measured at a frequency separation f from the carrier in a bandwidth BW.
Although the subject system provides the basis for new high performance wide band PM noise measurement systems, it does not provide a direct way to verify the performance of already existing PM noise measurement systems. Nor does it address the accuracy of amplitude modulation (AM) noise measurements. AM noise is very important in specifying the realized PM noise performance due to AM to PM conversion which is present in virtually all systems. The specification of PM (and recently AM) noise is required for most sources requiring high levels of precision. There is also a demand for noise measurement systems which can be traceable to their primary calibration laboratory. To achieve traceability, it seems necessary to have a PM noise calibration standard that is small, portable, rugged, and has high accuracy and repeatability. Such a standard should measure the noise floor of the system and the accuracy of measuring a known level of PM noise over wide bandwidths.
Presently available noise sources have no carrier associated with them and so are not suitable for use as either AM or PM noise standards. The present way in which measurements are now verified is for several laboratories to measure the noise of a commercial oscillator. This is not very satisfactory because the PM and AM noise levels are generally a function of their environment (also running time) and therefore subject to change. This arrangement is cumbersome, expensive and time consuming.